This invention relates to an improved process for dimerization of acrylonitrile to predominantly straight-chain C.sub.6 dinitriles such as 1,4-dicyanobutene. More particularly, the invention relates to improvements in dimerization processes using organic phosphinite and/or phosphonite catalysts in the presence of a proton-donating solvent.
Dimerization of acrylonitrile to yield 1,4-dicyanobutenes by using organic phosphinite or phosphonite catalyst in the presence of a proton-donating solvent is described, for example, in U.S. Pat. Nos. 4,126,632; 4,102,915; and 4,138,428, the disclosure of said patents being incorporated herein by reference. According to the teachings of these patents the acrylonitrile is contacted with an organic phosphorus (III) compound which has at least one hydrocarbyl and at least one alkoxy or cycloalkoxy group attached to the phosphorus atom or atoms, the acrylonitrile being dissolved in an organic solvent capable of donating protons and the acrylonitrile and solvent being substantially dry.
Suitable organic phosphorus (III) compounds include those of the formulae: ##STR1## where R.sub.1 is a hydrocarbyl group, R.sub.2 is an alkoxy or cycloalkoxy group, R.sub.3 is hydrocarbyl, alkoxy or cycloalkoxy group or other monovalent radical, and R.sub.4 is a divalent hydrocarbyl, hydrocarbyloxy or other difunctional group. It is also possible that one or more groups R.sub.1 to R.sub.3 may form part of one or more ring systems. The hydrocarbyl groups may be aryl, alkyl, alkylaryl (polycyclic) or cycloalkyl.
The reaction is conducted in the presence of an organic solvent since in the absence of solvent rapid polymerization of the acrylonitrile occurs. Suitable solvents are proton donating solvents which are substantially unreactive in respect of addition to, or reaction with, the unsaturated linkage of the acrylonitrile or the products of acrylonitrile dimerization. Furthermore, the solvent must not react with the phosphorus compounds or catalytic intermediates to form inactive phosphorus species at such a rate as to seriously impair the dimerization reaction. For example, phenols have been found to be unsuitable in this respect.
Preferably, hydroxylic solvents, such as alcohols, are used, provided that they do not react adversely with the phosphorus compound or any intermediates it may form with acrylonitrile. This may be readily established by experiment. Tertiary and secondary alcohols are preferred, for example, t-butylalcohol, 2-butanol and isopropanol.
The concentration of proton-donating solvent is generally in the range 5 to 50% by volume, calculated on the total volume of the reactants, but the optimum concentration will vary with the precise nature of the solvent and the catalyst compound. The molar concentration of proton-donating solvent will generally be greater than the molar concentration of the phosphorus (III) compound.
It is further taught that in order to reduce the amount of hexamer and/or other oligomers or polymers (hereafter referred collectively as polymeric by-products or merely polymers) which may be co-produced with the desired dimeric products, it is often desirable to add an inert, non-hydroxylic co-solvent to the reaction mixture used in the process. The co-solvent is dried to a level which maintains the overall anhydrous state of the system.
Suitable non-hydroxylic organic co-solvents include hydrocarbons, for example, hexane, cyclohexane, toluene, and petroleum ethers; ethers, for example, tetrahydrofuran, diethyl ether and diisopropyl ether; and nitriles, for example, acetonitrile, propionitrile: and fluorobenzenes. The hydrocarbon co-solvents are generally preferred. To avoid ambiguity, inert non-hydroxylic co-solvents such as described will, hereafter, be consistently referred to as "co-solvents" to clearly distinguish them from proton donating solvents employed in the reaction mix.
The reaction is conducted in the substantial absence of water.
The unpurified product of reactions of this type contains residual dimerization catalyst. This residual catalyst causes further reaction which increases the proportion of undesired byproducts. Even after purification the product may contain very small amounts of catalyst which surprisingly, even at very low levels, cause continued degradation or poison catalysts used in subsequent hydrogenation processes used to convert the dicyanobutene to adiponitrile. Clearly, any reduction of such problems associated with residual catalyst would represent an advance in the art.